Based on work done by us and others, appropriately-timed bright light exposure is now considered a viable treatment for circadian-phase disorders. These include jet lag, maladaptation to shift work and certain types of sleep disorders, and possibly certain types of mood disorders such as winter depression. Recently, we have discovered that a physiological dose of appropriately-timed administration of exogenous melatonin (M) can shift circadian rhythms in sighted humans. M administered in the morning delays circadian rhythms (shifts them to a later time); M administered in the evening advances circadian rhythms (shifts them to an earlier time). Thus, circadian phase disorders can potentially be treated with appropriately-timed M, alone or in conjunction with appropriately-timed bright light exposure. This is of particular importance in the elderly, who have difficulty with circadian phase disorders and who also are particularly sensitive to the side effects of medications. The dose of M effective for causing phase shifts is only 0.5 mg, which produces physiological levels. M is normally produced during the night; it is most effective in causing phase shifts when administered during the day. Therefore, we plan to administer a substance at the same levels it is normally produced in humans. Thus, treatment with M should be exceedingly safe for the elderly. In delineating the M phase response curve in elderly humans, we should be performing an essential first step in making M administration a viable treatment for circadian phase disorders in this population of individuals.